1. Field of the Invention
The present invention relates to an exposure method and an exposure apparatus, and a device manufacturing method.
2. Description of the Related Art
An exposure apparatus is an apparatus that transfers a pattern of an original (reticle or the like) onto a photosensitive substrate (e.g., wafer or the like where the surface thereof is coated with a resist layer) via a projection optical system in a lithography step included in manufacturing steps for a semiconductor device, a liquid crystal display device, and the like. One of the important optical characteristics of such an exposure apparatus is alignment accuracy with which the patterns in a plurality of steps are overlaid accurately. An important factor that influences the alignment accuracy includes a magnification error of the projection optical system. The required pattern size shrinks every year, and accordingly a requirement for an improvement in alignment accuracy also increases. Hence, it is very important to maintain the magnification of the projection optical system at a predetermined value. Here, it is known that, when the projection optical system absorbs a part of the exposure energy, a heat generated by the absorption changes the temperature of the projection optical system, resulting in change in the optical characteristics such as the refractive index of the projection optical system, that is, resulting in occurrence of so-called thermal aberration (exposure aberration). In particular, when the projection optical system is irradiated with exposure light over a long period of time, the imaging characteristics (e.g., focus, magnification, distortion aberration, astigmatism aberration, and wavefront aberration) of the projection optical system fluctuate. Consequently, non-negligible measurement errors may undesirably occur during alignment measurement.
In view of this, Japanese Patent Laid-Open No. S63-58349 discloses a projection optical apparatus that calculates the fluctuation amount of the imaging characteristics depending on the exposure energy irradiation state of the projection optical system using a model expression including the exposure amount, exposure time, non-exposure time, and the like as parameters to thereby correct the fluctuation in imaging characteristics of the projection optical system based on the calculation result. Here, the model expression has coefficients for the respective imaging characteristics specific to the projection optical system. The fluctuation in the imaging characteristics of the projection optical system can be calculated by setting the coefficients appropriately. In addition, Japanese Patent Laid-Open No. 2009-32875 discloses a method in which exposure under the second exposure condition is continuously performed prior to complete settlement of aberration fluctuation generated under the first exposure condition, and aberration fluctuation coefficients for predicting aberration fluctuation under the respective exposure conditions are determined so as to efficiently predict the fluctuation in the imaging characteristics for each exposure condition.
However, in the method disclosed in Japanese Patent Laid-Open No. S63-58349, when a coefficient is independently calculated for each exposure condition, the projection optical apparatus needs to be left to stand (cooled) until the aberration fluctuation generated under the first exposure condition almost disappears in order to increase determination accuracy. Thus, when many exposure conditions need to be taken into consideration, much time is required until aberration fluctuation is settled due to the characteristics of the thermal relaxation phenomenon. While, in the method disclosed in Japanese Patent Laid-Open No. 2009-32875, exposure under the second exposure condition starts prior to complete settlement of aberration fluctuation generated under the first exposure condition, the start of exposure under the second exposure condition is determined based on whether or not the preset required time has elapsed or whether or not a preset required number of measurement points has been acquired. In other words, in the method, after elapse of the required time, exposure under the second exposure condition forcibly starts regardless of the measurement state under the first exposure condition. Thus, if an error is generated in the aberration fluctuation coefficient under the first exposure condition due to factors such as a measurement error and the like, the first exposure condition shifts to the second exposure condition with the error included therein. Consequently, it becomes difficult to determine the aberration fluctuation coefficient under the second exposure condition with high accuracy.